Apparatus for calibrating thermostats



F. G. LOGUE 2,724,961

APPARATUS FOR CALIBRATING THERMOSTATS Filed June 8, 1950 Nov. 29, 195?:

MAW 5' W4 hz United States Patent APPARATUS FOR CALIBRATING THERMOSTATS Francis G. Logne, Catonsville, Md., assignor to Proctor Electric Company, Philadelphia, Pa., a corporation of Pennsylvania Application June 8, 1950, Serial No. 166,891

4 Claims. (Cl. 731) The present invention relates to an apparatus for calibrating thermostats, and has as its main object the provision of a novel apparatus that will enable rapid, eflicient, and accurate mass-calibration of thermostats.

The invention is especially adapted to calibration of thermostats of the type which include a bimetal controlled contact and a normally stationary contact. tion involves essentially the forcible movement of the normally stationary contact until engagement is made between it and the bimetal controlled contact, and the termination of such movement after engagement of the contacts. In the preferred embodiment, the termination of said movement is effected at the end of a predetermined time interval following the engagement of the contacts.

Various features and further objects of this invention will be apparent from the following detailed description of the invention.

In the accompanying drawing:

Fig. l is an exploded isometric illustration of a thermostat adapted for calibration by the apparatus of the present invention;

Fig. 2 is a sectional view of the thermostat on a larger scale; and v Fig. 3 is a schematic diagram of the preferred circuit and apparatus according to this invention.

The calibration and the means for performing the calibration will be explained, by way of example, with reference to the calibration of a thermostat used for safety purposes in an electric blanket. This particular application requires that a great number of thermostats be accurately calibrated at approximately 145 deg. F. within tolerance of deg. F.

This thermostat, designated generally by reference numeral 1, is shown in Figs. 1 and 2 to comprise a support member 2 of U-shaped cross-section made preferably of brass, having a bimetal 3 fixed thereto by rivet members 4 and 5 but electrically insulated therefrom by members 6, 7 and 8. The bimetal is arranged with its high expansion side downward and has a contact 9 fixed to its free end for cooperating with a stationary contact 10 set into a hole 11 in the metallic support member 2.

The present invention is concerned with the setting of the stationary contact into a correct position corresponding to the desired separating temperature for the surfaces of contacts 9 and 10. The fit between the contact member 10 and the support member is of the interference type. For this particular device the contact 10 is .0005 larger than the diameter of hole 11. It was found that, when using a thermostat support case of brass and contacts of fine silver, too great an interference fit caused shaving'of the side'of the contacts with subsequent loosening thereof. Further, the contact 19 is provided with a tapered section 10a for facilitating its insertion. Fig. 2 shows in dotted outline theposition of the contact after being placed into hole 11 and prior to calibration.

A terminal member 12 is spot-welded to the support member 2 while a second terminal 13 is held in electrical The calibra- I connection with the bimetal 3 through the rivets 4 and 5. These rivets and the insulating strips 6, 7 and 8 act to hold the second terminal in non-varying mechanical relation to the support member 2 but electrically insulated therefrom. After calibration, a protective metallic cover member 14 is snapped into position over the active portion of the bimetal 3, there being a paper insulating member 15 disposed between it and the underside of the cover tional travel which pro-stresses the bimetal 3 to such an extent that it will require a temperature rise to a predetermined temperature to effect opening of the con-' tacts. Thus where the desired opening temperature of the thermostat is 145 degrees F., as previously stated, it is desired that a rise from room temperature (70 degrees F.) to 145 degrees F., i. e. a rise of degrees, will be required to cause opening of the contacts. The magnitude of the'additional travel can be determined experimentally or can be determined from considerations of the bimetal characteristics and desired opening temperature. Thus the required additional travel of contact 10 can be determined from published data such as that given on page 33 of the Wilco Blue Book issued by the H. A. Wilson Co.

of Newark, N. J. For example, with a bimetal element having a thickness of .030 inch and a length of .89 inch, it can be seen from the chart on said page that the deflection will be .0002 inch per degree F. 75 degree rise from room temperature, the required additional movement of contact 10 will be 75 x .0002 or .0150

inch.

The preferred apparatus and circuit for calibrating the thermostats, as shown schematically in Fig. 3, includes an anvil 16 on which the thermostat 1 is placed, a plunger 17 which engages the contact 10 of said thermostat, and a motor 18 for driving the plunger 17 through a clutch connection 19 and a cam member 20. A spring 21 holds the plunger 17 biased against the surface of the cam 20, while a spring 22 urges thefaces of the clutch 19 into engagement. A solenoid 23 is provided which when energized disengages the clutch faces through lever 23a to discontinue the driving of the contact 10 through the plunger 17. A spiral spring 24 is provided for returning the driving cam 20 to its initial position which is established by a stationary pin 24a engageable by a pin 20a on the cam shaft. It has been found satisfactory to provide the cam 20 with 300 of useful periphery and to use a constant rise of approximately .001" per 3 /2" of revolution.

The solenoid 23 is momentarily energized at the end of a timed cycle by a timer 25 and is maintained energized by a hold-in switch 26 actuated by the solenoid 23. The

timer 25 is of a type adapted to produce a current flow.

at the end of a timed interval. It is preferably of the electronic type using the inherent time delay effect of a resistance-capacitance combination. ple, the timer may be of the type manufactured and sold by the Allen-Bradley Co. under the designation.Bulletin 852. The external connection, as far as concerns the present invention, includes the initiating terminals 28 and 29 which when bridged start the timer, the output terminals 30, 31 across which a voltage is established after a time interval, and connections 32 and 33 to the power source. It will be understood, of course, that any timer capable of giving the required accuracy may be used. The

Patented Nov. 29, 1955 Therefore, for a By way of exam-.

hold-in circuit for the solenoid is energized directly from the supplyline and includes a pushbutton switch 27 which is normally biased closed. The time-cycle of the timer 25- is initiated upon engagement'of the thermostat contacts 9 and .10 which acts toenergize .relay 34 and thereby bridge the initiating terminals 28 and 29. The circuit for the thermostat contacts 9 and 10 includes the shell 2 of the thermostat, the anvil 16, a conductor 35, the coil of the relay 34, a connection wire 36 and switches 37 and 38 for disconnecting both sides of the supply line. The latter switches are operated by the solenoid 23 so as to leave the thermostat electrically safe after calibration. The plunger 17 is insulated by an insert 17a while the anvil is also suitably insulated. It has been found ad vantageous to use 11 volts across contacts 9 and 10 during calibration, substantial lower voltages being insufficient to break down oxide film on the contacts under calibration.

A typical cycle of calibration starts with the placing of a thermostat 1 on the anvil 16 and the establishing of electrical connection between the wires 35 and 36 and the shell 2 and-bimetal 3. The thermostat is placed so that the contact 10 aligns itself with the plunger 17, the contact 10 being placed into hole 11 prior to placing the assembly .under the plunger. Fig. 3 shows the elements of the circuit and calibrating apparatus disposed as they would be at the completion of a typical calibration cycle or prior to the initiation of a new calibration cycle. Under this conditioning the solenoid 23 will be energized through hold-in switch 26, switches 37 and 38 will be open thus keeping the anvil 16 and connection wire 36 deenergized, the clutch 19 will be open and the motor 18 will be running continuously. The automatic phase of the calibration cycle is then initiated by momentarily pressing the normally-closed switch 27'. Opening of this switch will cause deenergization of solenoid 23 with consequent opening of switch 26 and engagement of the faces of clutch 19 under action of spring 22. Rotation of cam 20 will 1 cause the plunger 17 to force the contact 10 through the hole '11 until contact 10 engages the contact 9 mounted at the end oif'bimetal 3. This establishes a current through the relay 34 which initiates the cycle of the timer 25. Aittcr a predetermined length of time, the timer causes energization of the solenoid 23 with consequent opening ofthe clutch 19 and stoppage of the downward movement of plunger 1'7 and return of cam Zllto its initial position'by'spring24. Movement of the solenoid'23 isaccompanied by opening-of switches '37 and 3S and closing of switch 26 which-actsto hold in the solenoid.

The particular time required in this instance is in the orderof'a fewtenths of a second. As mentioned previousl-y the-time required is atleast'a function of the bimetal characteristic and the required temperature rise. It is also obviously controlled by the rise on the cam which must necessarily be constant to make the system sensitive only to the differential in displacement of the plunger from the instant of engagement of contacts 9 and- 10 and -the -instant of stopping the forcing action. Thus no matter what the angular position of the -cam is: at the beginning of the forcing" action or at "the instant of engagement of' contacts 9 and 10, the additional angular displacement of the cam and the linear displacement of the-contacts will be the same and solely a function of 'the time elapsed from contactengagement. It is also necessary that the :motor be of constant-speed and the spring back in the thermostat be either negligible or consistent between various samplesor otherwise accounted for;

Theedrivingof the-contact through a hole in the thermostat support constitutes .the preferred procedure of producing. a permanent displacement between theend-of a normally stationary contact and a bimetal controlledcontact. Otherprocedures are,,how.e.ver,, evident. Thus itwould. be possible to have the stationary contactfixed to the support structure and the area of the support in the vicinity of the contact forcibly distorted beyond its elastic limit so as to impart a permanent set thereto and at the same time give a discrete displacement to the normally stationary contact.

While I have shown a particular embodiment of my invention and a particular use thereof, it will, of course, be understood that I do not wish to be limited thereto, since difierent modifications may be made in the circuit,

the various elements therein, and the use thereof within the scope of this invention.

I claim:

1. Apparatus for calibrating thermostatic switches of the type which include a bimetal controlled contact and a normally stationary contact arranged for movement during calibration, comprising means for moving the lastmentioned contact toward the first-mentioned contact at a uniform rate, timing means, means responsive to .engagement of said contacts to initiate operation of said timing means, and means controlled by said timing means to terminate the operation of said first means after a time interval.

2. Apparatus for calibrating thermostatic switches of the type which include a bimetal controlled contact and a normally stationary contact arranged for movement during calibration, comprising means including a rotatable cam for moving the last-mentioned contact toward the first-mentioned contact, a driving motor for said cam, a clutch between said motor and said cam, a solenoid normally energized to hold said clutch disengaged, manual means for deenergizing said solenoid to effect engagement of said clutch, thus causing said motor to commence driving said cam, timing means adapted when started to energize said solenoid after a desired time interval, and means responsive to engagement of said contacts to start said timing means.

3. Apparatus for calibrating thermostatic switches of the type which include a bimetal controlled contact and a normally stationary second contact arranged for movement during calibration, comprising means for supporting a thermostatic switch to be calibrated, means for moving said second contact toward the first-mentioned contact to effect engagement of the contacts and stressing of the bimetal, and means for stopping the movement of said second contact at a position thereof in which the bimetal is stressed an amount to require a predetermined temperature rise above the ambient calibrating temperature to elfect opening of the contacts.

4'. Apparatus for calibrating thermostatic switches of the type which include a bimetal controlled contact and a normally stationary second contact arranged for movement during calibration, comprising means for supporting .a thermostatic switch to be calibrated, means for moving said second contact at a uniform rate toward the firstmentioned contact to eflect engagement of the contacts and'stressing of the bimetal, means operable in response to engagement of said contacts to time the subsequent movement ofsaid second contact, and means for stopping the movement of said second contact at the end of a timed interval with the bimetal stressed an amount to require. a predetermined temperature rise above the ambient calibrating temperature to effect opening of the contacts.

References Cited in'the file of this patent UNITED STATES PATENTS.

Great Britain Nov. '11, 194.9 

